Enabled branch protection for master

[bjorn]

I've figured out where GitHub was hiding the option to require a review before merging changes. Turns out it was under "Branches" since these settings are per-branch.

So, now the master branch of rs-tiled is protected, with changes always requiring a pull request with at least one approval, which also passes the "build" check that we've set up (which checks compile and runs tests). I hope it won't get in the way. :-)

Another thing I'd like to suggest, is to prefer squashing pull requests when merging. This keeps the history of the branch much more readable. Merging without squashing will remain possible, but should only be used when there are multiple separate commits that we'd prefer to keep separate.